DE 101 24 275 B4 refers to a method and a measuring arrangement for the measurement of tools. The measuring arrangement includes a base plate with an adapting arrangement for adapting a tool that is to be measured and a coordinate slide that is installed at the base plate. The coordinate slide holds the measuring system to be geared towards the tool. The measuring system includes camera systems with connected units for image processing. The solution provided particularly refers to measuring arrangements, where several adapters are used for the position-defined arrangement of the tools in the measuring system to be able to measure several, if applicable, different tools in the measuring arrangement. In each case, the adapters include a basic module and one or multiple insertion modules that are allocated to the basic module. To be able to measure a tool, which is inserted into the measuring arrangement by means of such adapter, it is necessary to establish a definite relationship between the measurement coordinate system of the measuring device and a tool-specific tool coordinate system. The tool coordinate system shows a so-called logical zero-point, which cannot be recognized from the outside. Therefore, at the insertion modules there is in each case an auxiliary zero-point that is also known as a gauging edge (actually, a calibration edge). The auxiliary zero-point is defined by auxiliary coordinates that on the one hand indicate the radial displacement between the auxiliary zero-point and the rotation axis and on the other hand the axial displacement between the auxiliary zero-point and the logical zero-point. The technical design of the gauging edge or auxiliary zero-point is not specified, but is only shown in the drawing by a crosslines symbol.
DE 10 2004 018 968 A1 also refers to the measuring arrangement for measuring of tools as shown in DE 101 24 275 B4. It is proposed to read out the auxiliary coordinates as module information from a two-dimensionally-structured optically ascertainable medium.
DE 10 2006 011 814 A1 refers to a tool measurement device for measuring a tool in a tool holder. The tool holder has a logical zero-point, which is termed a holder point. The tool holder also has an auxiliary zero-point, which is termed a measuring point. The measuring point is established by two spheres that are fixed in two metallic cavities of the tool holder that are arranged horizontally and vertically displaced to each other. This measuring point is used for the calibration of the tool measuring device on the used tool holder and is also termed a calibration feature.
FIG. 1 shows a calibration feature according to the state of the art in detail view. In particular, the figure shows a calibration feature as in DE 10 2006 011 814 A1. The calibration feature is arranged at an upper outside edge of a tool holder 01. The tool holder 01 can, for example, include a spindle or an adapter installed on a spindle. The calibration feature includes a first sphere 02 and a second sphere 03, both of the same type. The first sphere 02 is arranged in a first cylindrical opening 04 in the tool holder 01. The second sphere 03 is arranged in a second cylindrical opening 06 in the tool holder 01 and is pressed against the first sphere 02 by a spring 07, so that the first sphere 02 and the second sphere 03 contact in a contact point 08. Glue is put between spheres 02 and 03 and optionally also in the cylindrical openings 04, 06 for permanent fixation of the spheres.
The second sphere 03 is arranged displaced to the first sphere 02. The horizontal displacement and the vertical displacement of the center of the second sphere 03, compared to the center of the first sphere 02, respectively depends on the radius of both spheres 02, 03. Thus, a virtual connecting line between the central points of both of the spheres 02, 03 inclines at an angle of 45° compared to the horizontal and at an angle of 45° compared to the vertical.
Only one part of the surface of both of the spheres 02, 03 form a segment of the external surface of the calibration feature, since both of the spheres 02, 03 are embedded in both of the openings 04, 06. Each of the spheres 02, 03 is embedded in the openings 04, 06 to that extent that only 25% of the spheres' circumference is available for the image recording. Also the used glue 05 narrows the optically accessible contour of the circle.
The calibration feature is designed for optical registration, and in particular a first circular arched outside edge 09 of the first sphere 02 and a second circular arched outside sphere 11 of the second sphere 03 serve for optical registration. Thus, in particular only those parts of the circular arched outside edges 09, 11 can be recognized that form an outside contour of the calibration feature. The outside edges of the spheres 02, 03 each range from near the transition to both of the openings 04, 06 in the tool holder 01 to near the contact point 08 between the spheres 02, 03. Thus, the angle at the central point of the contour-forming circular arched outside edges 09, 11 in each case is about 95° only. Because of the limited central angle, the position of the spheres 02, 03 is optically ascertainable only to a limited extent. The contour-forming outside edges 09, 11 that are to be ascertained are arranged in different distances to the optical axis of an optical measuring system for the optical identification of the calibration feature and thus they underlie different errors in the optical imaging. These optical errors can only be corrected to a limited extent and do therefore influence the measurement of the calibration feature negatively.
The calibration feature formed by the spheres 02, 03 has areas that are difficult to access in the range of its common contact point 08 as well as at the transitions to the tool holder 01 that are prone to dirt and cannot be easily cleaned. The remaining dirt leads to uncertainties in the optical identification of the calibration feature.
For establishing the calibration feature, the openings 04, 06 and the spheres 02, 03 have to be exactly arranged to each other, thus the establishment of the calibration feature is complex and the calibration feature is prone to damage.